1. Field of the Invention
The present invention relates to an optical gyro and a technique of driving the optical gyro. More particularly, it relates to a constitution of an optical gyro in which light having a constant wavelength is propagated simultaneously clockwise and counterclockwise through an optical propagation path such as an optical fiber in cooperation with a rotation axis and then a phase difference of the light based on Sagnac effect is detected to obtain a signal proportional to an angular velocity with respect to the rotation axis, a signal processing apparatus for the optical gyro and a method of driving a phase modulator used in the optical gyro.
2. Description of the Related Art
Various approaches to drive an optical gyro have been developed. As an example, a frequency shift optical fiber gyro is known, which includes a phase modulator and a pair of acousto-optic modulators provided in an optical system optically coupled to a propagation path of an optical fiber through which light is propagated simultaneously clockwise and counterclockwise, and a signal processing circuit for analogically processing a photoelectric output signal obtained from the optical system. The signal processing circuit includes a synchronization detecting circuit responsive to the photoelectric output signal and a number of oscillating circuits, e.g., a voltage controlled oscillating (VCO) circuit, for generating drive signals of the above modulators and carries out an analog demodulation of the photoelectric output signal.
As described later in detail, however, the driving technique employing the analog demodulation has a number of drawbacks. For example, when drive frequencies for the pair of acousto-optic modulators are not identical to each other, an equivalent error of an angular velocity occurs, so that it is impossible to obtain a signal in accurate proportion to the angular velocity. Also, where the drive signals for the modulators are produced by separate oscillating circuits, the drive frequencies easily fluctuate resulting in a lowering in a bias stability of the gyro. Furthermore, due to a fluctuation in a quantity of incoherent light of a light beam in the optical system, a fluctuation in a maximum phase shift of the phase modulator, or the like, characteristics of the gyro are deteriorated. A problem also occurs in that a detectable maximum angular velocity is limited to a narrow range.
As another example, an optical fiber gyro using only a phase modulation is known, which includes a phase modulator provided in an optical system as above mentioned and a signal processing circuit for effecting an analog demodulation of a photoelectric output signal obtained from the optical system. In this case, the signal processing circuit includes an analog multiplier responsive to the photoelectric output signal and a drive signal for the phase modulator and a number of filters, e.g., a low pass filter for filtering an output of the analog multiplier.
Since the signal processing circuit employs the analog demodulation as well as the above example, like problems occurs. For example, where offset voltages occurring in the analog multiplier and low pass filter fluctuate, the bias stability of the gyro is lowered. Other problems will be described later in detail.
Also, in a known method of driving a phase modulator used in an optical fiber gyro, the phase modulator is modulated at a certain resonant frequency defined by dimensions of a member constituting the phase modulator. On the other hand, the phase modulation is preferably carried out at an eigenfrequency of an optical propagation path to remove influences due to harmonics contained in the phase modulation signal. Accordingly, it is necessary to conform the resonant frequency to the eigenfrequency.
As will be described later, however, the work imposes restrictions on the dimensions of the member constituting the phase modulator. As a result, where an optical fiber is wound on the member on which such restrictions are imposed, a radiation loss occurs in the optical fiber resulting in a deterioration in a signal-to-noise (S/N) ratio of a photoelectric output signal. Also, due to an error in manufacture of the member, it becomes difficult to conform the resonant frequency to the eigenfrequency.